This two‐part study describes the development and validation of a method for quantifying adolescent personality pathology using the latest edition of the Shedler–Westen Assessment Procedure for Adolescents (SWAP‐II‐A), an instrument designed to be used by clinically experienced observers. In Study 1, experienced psychologists and psychiatrists described a normative clinical sample of 950 North American patients. Study 2 applied the SWAP‐II‐A in a day treatment setting. Results indicated that SWAP‐II‐A personality disorder (PD) scales evidenced high internal consistency, construct validity with Diagnostic and Statistical Manual of Mental Disorders (5th ed.) symptoms and diagnoses, and concurrent validity with Child Behavior Checklist (CBCL) ratings. Independent observers saw patients similarly, and PD assessments were significantly associated with CBCL scale scores and ward behavior.